Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase

Metal clusters featuring carbon and sulfur donors have coordination environments comparable to the active site of nitrogenase enzymes. Here, we report a series of di-iron clusters supported by the dianionic yldiide ligands, in which the Fe sites are bridged by two μ2-C atoms and four pendant S donors.The [L2Fe2] (L = {[Ph2P(S)]2C}2–) cluster is isolable in two oxidation levels, all-ferrous Fe2II and mixed-valence FeIIFeIII. The mixed-valence cluster displays two peaks in the Mössbauer spectra, indicating slow electron transfer between the two sites. The addition of the Lewis base 4-dimethylaminopyridine to the Fe2II cluster results in coordination with only one of the two Fe sites, even in the presence of an excess base. Conversely, the cluster reacts with 8 equiv of isocyanide tBuNC to give a monometallic complex featuring a new C–C bond between the ligand backbone and the isocyanide. The electronic structure descriptions of these complexes are further supported by X-ray absorption and resonant X-ray emission spectroscopies.


General considerations
All manipulations were performed in an argon-filled MBraun glovebox maintained below 1 ppm of O 2 and H 2 O or under an N 2 atmosphere using standard Schlenk techniques unless mentioned otherwise. Glassware was oven-dried at 150 °C for at least 12 h prior to use. Celite and molecular sieves were dried above 200 °C under vacuum for at least 12 h. Pentane, THF, hexanes, benzene, toluene, and diethyl ether were purified by passage through activated alumina and Q5 columns from Glass Contour Co, under argon, and stored over activated molecular sieves. Benzene-d 6 and THF-d 8 were vacuum transferred from a solution of potassium benzophenone ketyl and was stored over 4 Å molecular sieves. LLi 2 , 1 Ferrocenium BAr F (Fc[{3,5-(CF 3 ) 2 C 6 H 3 } 4 B]), 2 and FeCl 2 (THF) 1. 5 3 were prepared by literature procedures. tert-Butyl isocyanide was purchased from Sigma Aldrich, degassed, and stored over molecular sieves prior to use. 4-Dimethylaminopyridine (DMAP) was purchased from ThermoFisher and used as received.
NMR data were collected on Agilent 400 or 500 MHz spectrometers. Chemical shifts in 1 H NMR spectra are referenced to the residual proton solvent peaks of C 6 D 5 H (δ 7.16 ppm), THF-d 8 (δ 3.58 ppm), and CDHCl 2 (5.32 ppm). Elemental analyses were performed at the CENTC Elemental Analysis Facility at the University of Rochester. IR spectra were collected on an Alpha Platinum ATR IR Spectrometer. UV-vis spectra were recorded on a Cary 50 spectrometer using Schlenkadapted quartz cuvettes with a 1, 2, or 10 mm path length.

Modified synthesis of [LFe] 2 , 1 4
Crystals of LLi 2 (250 mg, 0.543 mmol) were dissolved in THF (5 mL) and cooled to -78 °C. Solid FeCl 2 (THF) 1.5 (128 mg, 0.543 mmol) was suspended in THF and added dropwise to the solution of LLi 2 . The remaining FeCl 2 (THF) 1.5 was rinsed from the vial with THF (3 × 1 mL) and added to the reaction mixture. After a few minutes of stirring, the reaction turned dark brown. The reaction mixture was allowed to warm to room temperature and stirred overnight. The solvent was removed, and the resulting solids were taken into benzene (30 mL) and filtered through a medium porosity glass frit. The remaining solids were washed with benzene (3 mL portions) until the filtrate ran colorless. The filtrate was then dried under vacuum and washed with THF (2 × 5 mL), Et 2 O (2 × 5 mL), and hexane (2 × 5 mL) and dried under vacuum to give 1 as a green powder.

Synthesis of [LFe(DMAP)FeL], 3
A solution of 4-dimethylaminopyridine (14.5 mg, 0.118 mmol) in benzene (0.5 mL) was quickly added to a saturated solution of [LFe] 2 (70 mg, 0.07 mmol) in benzene (18 mL) resulting in an immediate color change from green to red. The reaction mixture was shaken for five seconds and was stored for 24 h at ambient temperature resulting in the formation of some red crystals. nhexane (2 mL) was added and the reaction mixture was held for 48 h to give additional red crystals.

Synthesis of 4
To a stirring solution of 1 (118 mg, 0.118 mmol) in toluene (3 mL) was added a solution of t BuNC (80 mg, 0.962 mmol) in toluene (1 mL). The solution immediately turned from green to red. The solvent was removed, and the resulting red oil was washed with hexane (3 × 2 mL) to leave an orange solid. The remaining solid was taken into THF (4 mL) and filtered. The dark yellow solution was concentrated to 1 mL and 10 drops of hexane were added. Cooling this solution to -35 °C overnight yielded yellow crystals of 4 (86 mg, 44%).

Mössbauer spectra and assignments
Mössbauer spectra were recorded on a conventional spectrometer with an alternating constant acceleration of the γ-source. The minimum experimental line width was 0.24 mm/s (full width at half height). The sample temperature was maintained constant in an Oxford Instruments Variox cryostat. The 57 Co/Rh source (0.6 GBq) was kept at room temperature. Isomer shifts are quoted relative to iron metal at 300 K. Zero field spectra were measured at 80 K.

Magnetic measurements
Magnetic susceptibility data were measured from powder samples of solid material in the temperature range 2-290 K by using a SQUID magnetometer with a field of 1.0 T (MPMS-7, Quantum Design, calibrated with a standard palladium reference sample, error < 2%). Sample holders of quartz with an O-ring sealing were used. The SQUID response curves (raw data) have been corrected for holder and solvent contributions by subtracting the corresponding response curves obtained from separate measurements without sample material. In addition, the experimental magnetization data obtained from independent simulation of the corrected SQUID response curves were corrected for underlying diamagnetism by use of tabulated Pascal's constants, as well as for temperature-independent paramagnetism (TIP). 5 Magnetic susceptibility data were analyzed and simulated using the julX software developed by E. Bill (Max Planck Institute for Chemical Energy Conversion, Mülheim an der Ruhr).

X-ray Absorption (XAS) and Emission (XES) Spectroscopy Methods
Resonant and non-resonant Kβ mainline XES and high energy resolution fluorescence detected (HERFD) XAS measurements were performed at beamline 6-2 of the Stanford synchrotron radiation lightsource (SSRL). Partial fluorescence yield (PFY) XAS measurements were performed at beamline 9-3 of the SSRL synchrotron. Measurements at both beamlines were performed according to the previously reported procedure. 7-9 XAS spectra were calibrated using an iron foil reference, with the first inflection point set to 7111.2 eV. XES spectra were calibrated by setting the maximum of the Kβ 1,3 line of an Fe 2 O 3 reference to 7060.6 eV.
Raw PFY-XAS data were pre-processed via the ATHENA program, part of the Demeter software package. 10 Partial fluorescence yield (PFY) detected XAS scans were averaged together, and further pre-and post-edge background subtraction and the edge jump normalizations were applied.
. Prior to normalization of the resonant and non-resonant XES spectra to the Kβ 1,3 -maximum (set to one as previously published 12 ), an offset correction was performed by averaging emission intensities starting from 7080 eV was applied using the following Python code (Code 1).
Code 1: Python function for converting XES into normalized XES spectra by using the Pandas library package for dataframes Numeric operations and plots are generated via the Python packages Pandas v1.4.3, NumPy v1.23.1, and matplotlib v3.5.2. [13][14][15] The first derivative of the 1, 2, and 3 are generated via numpy.gradient 1 module (Code 2) with non-homogeneous stepsize. The numerical gradient is calculated to the 1st order at the boundary condition, generally the approximation of is .